Faba bean is a poor competitor, so good weed control is essential and often the most important consideration for profitable faba bean production. Try to control weeds using all available methods by adopting an integrated weed management (IWM) strategy that combines both chemical and cultural practices.
The major weeds of concern in faba bean crops are Canada thistle and perennial sow thistle. Other significant weeds include: cow cockle, roundleaved mallow, bluebur, and wild tomato.
Weed Seedling Identification
Good weed management begins with knowing what weeds are present. Fields should be inspected first thing in the spring as this is an ideal time to control winter annuals. The next weed inspection should be just prior to crop emergence to time pre-emergent burn-off. Fields should be inspected immediately upon crop emergence to establish the frequency and distribution of weed species to determine the appropriate herbicide products. Follow-up scouting should occur as the crop approaches the optimum stage for post-emergent weed control.
Herbicide Resistant Weeds
Weed populations contain a great deal of natural variability to survive changing environmental conditions. Herbicides select for individual plants with the ability to survive the herbicide treatment. Surviving plants then contribute to a larger proportion of the weed population. It is important to realize that selection for herbicide resistant weeds occurs each time a herbicide is applied.
Herbicides have different modes of action. Some modes of action are easy for weeds develop resistance to as it only requires variation in a few genes (high risk of resistance), while others may require changes in multiple genes (low risk of resistance). Weeds that have developed resistance in the Prairies are listed at: http://www.weedscience.org. For faba bean production the following mode of actions are currently available:
Table 1. Herbicide Modes of Action Currently Registered for Faba Bean
|Herbicide Group*||Herbicide||Application Time||Weeds Controlled|
|Group 1||Poast® Ultra||Post-emergent||Grassy weeds|
|Group 2||Express® SG||Pre-seeding||Select broadleaves|
|Group 3||Edge™ Granular Trifluralin||Pre-plant incorporated||Grass and broadleaf weeds|
|Group 5||Sencor® (combined with Trifluralin)||Pre-plant incorporated||Select broadleaves|
|Group 6||Basagran®/Basagran® Forté||Post-emergent||Select broadleaves|
|Group 9||Glyphosate||Pre-seeding, pre-emergent, pre-harvest & post-harvest||Broad spectrum|
Note: Check product labels for herbicide group number(s) to confirm the mode of action as new products and mixtures are always being introduced.
Group 1 and group 2 herbicides have a high risk of developing resistant weed populations, therefore, the longer you can rotate away from these chemistries the better. If a grower has applied Group 1 or 2 herbicides more than ten times in a field, there is a high risk of resistance developing among one or more weed species.
Before applying herbicides consider the field’s herbicide history and follow an appropriate herbicide group rotation to avoid the development of herbicide-resistant weeds.
Herbicide-resistant weeds are more likely to occur under the following conditions:
- High weed number
- Too frequent use of a particular herbicide group or mode of action
- Not using recommended rates
- Allowing surviving weeds to set seed
Resistance is not reversible. That means stopping the use of the herbicide will not change the frequency of resistance in the weed population. However, research indicates that alternating between two modes of action for wild oat control will double the number of years for resistance build-up, and alternating with a third mode of action will quadruple the time of resistance build-up (Cavan et al., 2001).
Cultural Weed Control
There are several methods of cultural weed control:
- Use clean, weed-free seed
- Use diverse crop rotations
- Good sanitation practices, such as cleaning harvest and seeding equipment to avoid spreading weed seeds between fields
- Seed early. Faba bean plants are more competitive with annual weeds if they emerge rapidly and cover the soil surface before the weeds emerge
- Spring tillage, even minor tillage, significantly increases the burial and resulting germination of false cleavers and catchweed bedstraw. However, growers should limit spring tillage as part of an integrated weed management program (Reid et al., 2005).
- Make the crop more competitive (i.e. higher seeding rates) to help choke out weeds and help reduce the reliance on herbicides
The decision to spray or not to spray should be based on economics. If the potential yield loss is greater than the cost of the chemical and application, then you should spray. Consult the current Saskatchewan Ministry of Agriculture's Guide to Crop Protection for examples of economic thresholds and situations where the choice to spray or re-spray may not be clear.
Prior to spraying, producers should consider the following spray tips:
- Evaluate sensitivity of the surrounding environment and avoid spray drift
- Inform your neighbors about the location of the faba bean crop
- During periods of crop stress (heat, drought, frost, or after land rolling), the ability of the faba bean crop to tolerate herbicides may be reduced. Crop injury can be reduced by:
- waiting approximately four days after the crop stress occurs before applying a herbicide
- maintaining water volumes at label recommendations
- Applying the herbicide in the evening
- Thoroughly clean sprayers according to the label directions of the previous herbicide, as very small amounts of residue of some herbicides can cause significant crop injury
- Ensure your sprayer is functioning properly (leaks, plugs, pressure gauge, etc.) and choose appropriate nozzles for the situation
Pre-plant Herbicide Options
Prior to planting your faba bean crop, ethalfluralin (Edge®) or trifluralin can be used to control some annual broadleaf and grassy weeds. These Group 3 herbicides are registered for pre-plant incorporated use in the fall or spring on fields to be seeded to faba bean. Faba bean is also tolerant of pre-plant incorporated Metribuzin (Sencor®) and there are no restrictions on depth of seeding with the use of preplant incorporated Sencor®.
When using Trifluralin the following factors should be considered:
- Trifluralin application rates depend on soil organic matter and month of application
- Risk of injury to faba bean may occur if soil and weather conditions are not conducive to rapid crop emergence (cold or dry soils at time of seeding)
- Sencor can be pre-plant incorporated (tank mixed with Treflan EC)
When using Ethalfluralin (Edge™) the following factors should be considered:
- Edge™ should not be applied on peat soils
- Edge™ application rates depend on soil organic matter content
- To reduce the risk of injury, use good-quality certified seed
- Edge™ suppresses cleavers, including Group 2 resistant cleavers, which is useful when using Basagran® in crop, as Basagran® offers only fair control of cleavers
Pre-emergent Herbicide Options
Faba bean is slow to develop, so a pre-emergent herbicide burn-off is critical for successful faba bean production. It is important to remember that the timing window for pre-emergent herbicide applications is very narrow and will vary according to depth of seeding and growing conditions. The application window may be only a few days under ideal conditions and shallow seeding. It may be necessary to stop seeding to apply your pre-emergent burn-off in a timely manner.
The current herbicide products registered for use on faba bean prior to emergence include glyphosate and tribenuron-methyl (Express® SG). Products containing phenoxy herbicides such as dicamba or 2, 4-D should not be used before or just after planting.
Post-emergent Herbicide Options
Post-emergent herbicides should be applied at the earliest crop stage listed on the herbicide label. For Basagran®, this would be 10 cm (4 in) height, while Poast Ultra does not list a leaf stage restriction. Spraying early will minimize crop damage. Crop injury due to late herbicide application is a common problem.
Early herbicide applications also result in the best weed control. To maximize yield potential, spray once most grassy weeds have germinated. Weed seedlings are easiest to control at a young stage.
If you have to spray at later crop stages, remember to follow pre-harvest interval restrictions on the label. For example, Poast Ultra has an 85-day pre-harvest interval.
Always follow label recommendations and check product labels carefully. Your most up-to-date label information is available on the manufacturer’s web site links to the product label.